The invention relates to an assembly unit consisting of a belt retractor for a vehicle seat belt wound on a belt reel of the belt retractor, a belt tensioner drive providing a drive energy by means of which the belt reel of the belt retractor can be driven in the winding direction, and a gearing coupling the belt tensioner drive to the belt reel, as is known from EP-A-0 649 779 or EP-A-0 628 454. The belt tensioner drive is activated when required by known ways and means to eliminate the so-called belt slack in the seat belt system, thus ensuring that a vehicle occupant is involved as early as possible in the deceleration of the vehicle in a crash situation.
In conventional belt tensioner drives, a pyrotechnic charge is employed which can be ignited and provides compressed gas acting on either a translatory shiftable piston or a rotary piston. The movement of the piston is transmitted in a suitable way to the belt reel so that the latter is turned in the winding direction. A gearing may be arranged between the tensioner drive and the belt reel.
It is problematic in the case of known belt tensioners that on commencement of the tensioning action, i.e. shortly after activation of the pyrotechnic charge, the components involved in the tensioning action are subjected to extremely high accelerations. Due to the mass inertia forces acting in this case which then need to be overcome, the components involved in transmitting the force from the piston to the belt reel are made of high-strength steel. This is a drawback as regards both weight and cost.
The invention provides an assembly unit consisting of a belt retractor and a belt tensioner drive, in which the loads on the components in the course of belt tensioning can be influenced in such a way that extreme values are avoided. This is achieved in accordance with the invention by the gearing being a gearwheel system comprising a driving gear and a driven gear and providing a translation ratio which changes during a tensioning action. In such a gearing a variable translation is achievable which can be adapted to individual requirements by employing toothing pitch circles deviating from the conventional round shape. Providing for a variable translation permits influencing the conversion of the drive energy made available by the pyrotechnic charge into rotation of the belt reel in such a way that the loads on the components involved in transmitting the force are significantly reduced. It is particularly of advantage when the translation ratioxe2x80x94starting from the value existing at commencement of the tensioning actionxe2x80x94increases at least initially during the tensioning action. Due to the initially comparatively low translation ratio, a particularly low loading of the components involved in the force transmission is achieved at commencement of belt tensioning. The later increase in translation, however, ensures that in effort at least the same tensioning performance as in conventional systems is achieved. Reducing the loading on the components enables light metal or even plastics to be used as the material for the gearwheels used in the gearing, this resulting in, apart from weight savings and reductions in cost, also reduced mass inertia which further improves tensioning performance.
In accordance with a preferred alternative embodiment of the invention it is provided for that between the belt tensioner drive and the belt reel, the gearwheel system and a rack and pinion gear are arranged coupled in series, the rack and pinion gear consisting of a rack forming the input side of the gearing and of a pinion forming the output side of the gearing, that the toothing of the rack is curved relative to its direction of movement and that the pinion is eccentrically mounted with regard to the pitch circle of its toothing. Such a rack and pinion gear which basically is known from U.S. Pat. No. 5,397,075, permits by particularly simple means to transmit the drive energy made available, for example, by a linear drive, with a variable translation ratio to the belt reel. The rack may be configured to advantage on the outer surface area of the piston of such a linear drive. By combining the rack and pinion gear with the gearwheel system, each of which providing a variable translation ratio, the translation ratio as a whole can be adapted to the individual requirements in an even larger range.
Further advantageous aspects of the invention read from the sub-claims.